Concave cavity for integrated microfabricated sensor

ABSTRACT

An integrated microfabricated sensor includes a sensor cell having a cell body, a first window attached to the cell body, and a second window attached to the cell body. The cell body laterally surrounds a cavity, so that both windows are exposed to the cavity. The sensor cell contains a sensor fluid material in the cavity. The cavity has concave profiles at cell body walls, so that the cavity is wider in a central region, approximately midway between the first window and the second window, than at the first surface and at the second surface. The cell body walls of the cell body have acute interior angles at both windows. The cell body is formed using an etch process that removes material from the cell body concurrently at the first surface and the second surface, forming the acute interior angles at both the first surface and the second surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/457,669 filed Mar. 13, 2017, which is incorporated herein byreference in its entirety.

FIELD

This disclosure relates to the field of integrated microfabricatedsensors. More particularly, this disclosure relates to cavityarchitecture in integrated microfabricated sensors.

BACKGROUND

A sensor cell of an integrated microfabricated sensor commonly has asandwich structure with a cell body of crystalline silicon between twowindows, with a cavity extending through the cell body, between thewindows. The sensor cell may be fabricated by etching the cavity in thesilicon using a crystallographic wet etch process which produces facetedcell body walls, so that the cavity is wider at one end than at theother end.

After the cavity is etched, the first window is attached. In oneapproach, the window is attached to the side of the cell body with thenarrow cavity end, so that the wide end of the cavity is open. Asolution of alkali metal salt, such as cesium azide dissolved in wateror alcohol, is dispensed into the cavity. The solvent is removed byevaporation. This approach has a problem with wicking of the solutiononto the exposed surface of the cell body where the second window isattached, because the surfaces are hydrophilic, and because the cellbody wall has an obtuse interior angle at the exposed surface,facilitating wicking onto the exposed surface. As the alkali metal saltprecipitates from the removal of the solvent, the wicking is exacerbatedby diffusion of the solution through the precipitated metal salt. Theprecipitated metal salt on the exposed surface of the cell bodyinterferes with the attachment of the second window. The cavity isapproximately filled with the solution, to attain a desired amount ofthe metal in the cavity. Increasing the concentration, to reduce thefill volume, exacerbates the wicking, and has not been effective insolving the problem. Making the cavity surfaces hydrophobic, to avoidwicking, tends to precipitate the metal salt on the middle area of thewindow, obscuring a signal path through the cell.

In another approach, the first window is attached to the other surfaceof the cell body, so that the cavity is narrower at the open end. Inthis approach, the solution tends to precipitate the metal salt in thecenter of the window. This undesired precipitation in the signal path isa result of the obtuse interior angle of the cell body wall at thesurface abutting the window.

SUMMARY

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the disclosure. This summary isnot an extensive overview of the disclosure, and is neither intended toidentify key or critical elements of the disclosure, nor to delineatethe scope thereof. Rather, the primary purpose of the summary is topresent some concepts of the disclosure in a simplified form as aprelude to a more detailed description that is presented later.

An integrated microfabricated sensor includes a sensor cell having acell body, a first window, and a second window. The cell body has afirst surface, which is flat, and a second surface, which is also flat,parallel to the first surface and located on an opposite side of thecell body from the first surface. The cell body laterally surrounds acavity which extends from the first surface to the second surface. Thefirst window is attached to the first surface and extends across thecavity, so that the first window is exposed to the cavity. The secondwindow is attached to the second surface and extends across the cavity,so that the second window is also exposed to the cavity. The sensor cellcontains a sensor fluid material in the cavity. The cavity has concaveprofiles at cell body walls, so that the cavity is wider in a centralregion, approximately midway between the first window and the secondwindow, than at the first surface and at the second surface. The cellbody walls of the cell body have acute interior angles at both windows.

The cell body is formed using an etch process that removes material fromthe cell body concurrently at the first surface and the second surface,forming the acute interior angles at both the first surface and thesecond surface. The first window is attached, and a solution of metalsalt in a solvent is dispensed into the cavity. The solvent is removedby evaporation. The second window is subsequently attached.

BRIEF DESCRIPTION OF THE VIEWS OF THE DRAWINGS

FIG. 1 is an exploded view of an example integrated microfabricatedsensor.

FIG. 2A through FIG. 2H are views of an integrated microfabricatedsensor, depicted in successive stages of an example method of formation.

FIG. 3 is an exploded view of another example integrated microfabricatedsensor.

FIG. 4A through FIG. 4F are views of an integrated microfabricatedsensor, depicted in successive stages of another example method offormation.

DETAILED DESCRIPTION

The present disclosure is described with reference to the attachedfigures. The figures are not drawn to scale and they are provided merelyto illustrate the disclosure. Several aspects of the disclosure aredescribed below with reference to example applications for illustration.It should be understood that numerous specific details, relationships,and methods are set forth to provide an understanding of the disclosure.One skilled in the relevant art, however, will readily recognize thatthe disclosure can be practiced without one or more of the specificdetails or with other methods. In other instances, well-known structuresor operations are not shown in detail to avoid obscuring the disclosure.The present disclosure is not limited by the illustrated ordering ofacts or events, as some acts may occur in different orders and/orconcurrently with other acts or events. Furthermore, not all illustratedacts or events are required to implement a methodology in accordancewith the present disclosure.

The following co-pending patent application is related and herebyincorporated by reference: U.S. patent application Ser. No. 15/457,608,filed simultaneously with this application). With its mention in thissection, this patent application is not admitted to be prior art withrespect to the present invention.

An integrated microfabricated sensor which may be, for example, anintegrated microfabricated atomic clock (MFAC) or an integratedmicrofabricated atomic magnetometer (MFAM), includes a sensor cellhaving a cell body, a first window, and a second window. The cell bodyhas a first surface, which is flat, and a second surface, which is alsoflat, parallel to the first surface and located on an opposite side ofthe cell body from the first surface. The cell body laterally surroundsa cavity which extends from the first surface to the second surface. Thefirst window is attached to the first surface and extends across thecavity, so that the first window is exposed to the cavity. The secondwindow is attached to the second surface and extends across the cavity,so that the second window is also exposed to the cavity. The sensor cellcontains a sensor fluid material, for example a material includingcesium or rubidium, in the cavity.

The cavity has cell body walls with concave profiles, wherein the cellbody walls have a first boundary region along the first surface, asecond boundary region along the second surface, and a central regionbetween the first surface and the second surface, so that the cavity iswider in the central region, than at the first boundary region and atthe second boundary region. The cell body walls of the cell body haveacute interior angles at both the first surface and the second surface,that is, a first interior angle from the first surface through the cellbody to the cell body wall at the first surface is less than 90 degrees,and a second interior angle from the second surface through the cellbody to the cell body wall at the second surface is less than 90degrees. The concave profiles may include planar facets, and may includecurved surface segments.

The integrated microfabricated sensor includes a signal emitter locatedproximate to the first window or the second window. In one context ofthe instant disclosure, the signal emitter being proximate to the firstwindow or the second window may be manifested by the signal emitterbeing located within a few millimeters of the first window or the secondwindow and facing the first window or the second window. In anothercontext, the signal emitter being proximate to the first window or thesecond window may be manifested by the signal emitter being locatedwithin a few millimeters of the first window or the second window andbeing configured to emit an input signal into the cavity through atleast one of the first window or the second window. The integratedmicrofabricated sensor further includes a signal detector locatedproximate to the first window or the second window. In one context ofthe instant disclosure, the signal detector being proximate to the firstwindow or the second window may be manifested by the signal detectorbeing located within a few millimeters of the first window or the secondwindow and facing the first window or the second window. In anothercontext, the signal detector being proximate to the first window or thesecond window may be manifested by the signal detector being locatedwithin a few millimeters of the first window or the second window andbeing configured to detect an output signal from the cavity through atleast one of the first window or the second window.

The cell body is formed using an etch process that removes material fromthe cell body concurrently at the first surface and the second surface,forming the acute interior angles at both the first surface and thesecond surface. The first window is attached, and a solution of metalsalt in a solvent is dispensed into the cavity. The solvent is removedby evaporation. The second window is subsequently attached.

For the purposes of this disclosure, the term “lateral” is understood torefer to a direction parallel to the first surface and the secondsurface the cell body. The term “vertical” is understood to refer to adirection perpendicular to the first surface and the second surface thecell body. The term “exterior” is understood to refer to lateralsurfaces of the cell body outside of the cavity.

It is noted that terms such as top, bottom, front, back, over, above,under, and below may be used in this disclosure. These terms should notbe construed as limiting the position or orientation of a structure orelement, but should be used to provide spatial relationship betweenstructures or elements.

FIG. 1 is an exploded view of an example integrated microfabricatedsensor. The integrated microfabricated sensor 100 includes a sensor cell102, which includes a cell body 104 having a first surface 106 and asecond surface 108. The first surface 106 is flat. The second surface108 is also flat, and is parallel to the first surface 106. The secondsurface is located on an opposite side of the cell body 104 from thefirst surface 106. The cell body 104 laterally surrounds a cavity 110.The cavity 110 extends from the first surface 106 through the cell body104 to the second surface 108. The cell body 104 may include, forexample, primarily crystalline silicon.

A first window 112 is attached to the cell body 104 on the first surface106 and extends across the cavity 110, so that the first window 112 isexposed to the cavity 110. A second window 114 is attached to the cellbody 104 on the second surface 108 and extends across the cavity 110, sothat the second window 114 is also exposed to the cavity 110. The sensorcell 102 contains a sensor fluid material 116, for example cesium orrubidium, in the cavity 110. The sensor fluid material 116 may beprimarily in the form of a condensed state of a sensor fluid. The sensorfluid may be, for example, cesium vapor or rubidium vapor, and thecondensed state of the sensor fluid may be solid cesium or solidrubidium, respectively. Alternatively, the sensor fluid material 116 maybe a salt such as cesium azide (CsN₃). Other materials for the sensorfluid and the sensor fluid material 116 are within the scope of theinstant example.

The cell body 104 has cell body walls 118 with concave profiles facingthe cavity 110, wherein the cell body walls 118 have a first boundaryregion 120 a along the first surface 106, a second boundary region 120 balong the second surface 108, and a central region 120 c between thefirst surface 106 and the second surface 108, so that the cavity 110 iswider in the central region 120 c, than at the first boundary region 120a and at the second boundary region 120 b. In the instant example, thecell body walls 118 have facets extending to the first surface 106 andto the second surface 108. The cell body walls 118 have a first interiorangle 122 which is acute, extending from the cavity 110 through the cellbody wall 118 to the first surface 106, around a perimeter of the cavity110 at the first surface 106. The first interior angle 122 may vary invalue from point to point around the perimeter of the cavity 110, butremains acute at each point. Similarly, the cell body walls 118 have asecond interior angle 124, extending from the cavity 110 through thecell body wall 118 to the second surface 108, which is acute around theperimeter of the cavity 110 at the second surface 108.

The integrated microfabricated sensor 100 includes a signal emitter 126located outside of the sensor cell 102, proximate to the first window112. The integrated microfabricated sensor 100 further includes a signaldetector 128 located outside of the sensor cell 102, proximate to thesecond window 114. A signal path extends from the signal emitter 126through the first window 112, through the cavity 110, and through thesecond window 114, to the signal detector 128. The acute values of thefirst interior angle 122 and the second interior angle 124 mayadvantageously induce disposition of the sensor fluid material 116 onthe first window 112 or the second window 114 around the perimeter ofthe cavity 110 and thus out of the signal path at the first window 112and at the second window 114.

FIG. 2A through FIG. 2H are views of an integrated microfabricatedsensor, depicted in successive stages of an example method of formation.Referring to FIG. 2A, a body substrate 230 is provided which includesareas for cell bodies, including the cell body 204 of the integratedmicrofabricated sensor 200. The body substrate 230 may be, for example,a silicon wafer, 500 micrometers to 3 millimeters thick. The bodysubstrate 230 has a first surface 206 which is flat, and a secondsurface 208 which is also flat. The second surface 208 is parallel to,and located opposite from, the first surface 206.

In the instant example, a first etch mask 232 and a second etch mask 234are formed on the first surface 206 and the second surface 208,respectively. The first etch mask 232 covers areas on the first surface206 for cell body walls 218 and exposes areas for cavities 210 of thecell bodies. Similarly, the second etch mask 234 covers areas on thesecond surface 208 for the cell body walls 218 and exposes areas for thecavities 210.

The first etch mask 232 and the second etch mask 234 may be formed, forexample, by forming a layer of silicon dioxide 5 nanometers to 20nanometers thick concurrently on the first surface 206 and the secondsurface 208 by a thermal oxidation process, followed by forming a layerof silicon nitride 100 nanometers to 500 nanometers thick concurrentlyon the first surface 206 and the second surface 208 by a low pressurechemical vapor deposition (LPCVD) process or a hotwall atmosphericpressure chemical vapor deposition (APCVD) process. Subsequently, afirst temporary mask of photoresist, not shown in FIG. 2A, may be formedon the layer of silicon nitride on the first surface 206 by aphotolithographic process, and the silicon nitride and silicon dioxidemay be removed in areas exposed by the first temporary mask by a plasmaetch process, or a reactive ion etch (ME) process, using halogen andoxygen radicals, leaving the first etch mask 232 in place, followed byremoving the first temporary mask. After the first etch mask 232 is thusformed, a second temporary mask of photoresist, not shown in FIG. 2A,may be formed on the layer of silicon nitride on the second surface 208,and the silicon nitride and silicon dioxide may be removed in areasexposed by the second temporary mask, leaving the second etch mask 234in place, followed by removing the second temporary mask. Other methodsfor forming the first etch mask 232 and the second etch mask 234 arewithin the scope of the instant example. Silicon dioxide of the layer ofsilicon dioxide under the silicon nitride may be removed after thesilicon nitride on both the first surface 206 and the second surface 208is etched, for example, by a wet etch of an aqueous solution of bufferedhydrofluoric acid.

Referring to FIG. 2B, substrate material of the body substrate 230 isremoved in areas exposed by the first etch mask 232 and the second etchmask 234 by an etch process, for example by a wet etch bath 236 asdepicted. If the body substrate 230 is a silicon wafer, the wet etchbath 236 may include an aqueous alkaline solution such as potassiumhydroxide or tetramethyl ammonium hydroxide. The wet etch bath 236 mayremove silicon and undercut the first etch mask 232 and the second etchmask 234 to form faceted surfaces on [111] crystal planes. FIG. 2Bdepicts the body substrate 230 partway through the etch process. Theetch process of the instant example is continued until the substratematerial of the body substrate 230 is completely removed in the cavity210, and the profile of the body substrate 230 in the cavity 210 isreversed, to provide acute interior angles around perimeters of thecavity 210 at the first surface 206 and the second surface 208.

FIG. 2C depicts the body substrate 230 after the etch process of FIG. 2Bis completed and after the first etch mask 232 and the second etch mask234 of FIG. 2B have been removed. Silicon nitride in the first etch mask232 and the second etch mask 234 may be removed, for example, by a wetetch of an aqueous solution of phosphoric acid. Silicon dioxide in thefirst etch mask 232 and the second etch mask 234 may be removed, forexample, by a wet etch of an aqueous solution of buffered hydrofluoricacid. In the instant example, the cell body walls 218 have facetedconcave profiles, as a result of the crystallographic etch mechanism ofthe wet etch 236 of FIG. 2B. The cavity 210 is wider in a central region220, approximately midway between the first surface 206 and the secondsurface 208, than at the first surface 206 and at the second surface208. The cell body walls 218 have a first interior angle 222 at thefirst surface 206 which is acute around a perimeter of the cavity 210 atthe first surface 206. Similarly, the cell body walls 218 have a secondinterior angle 224 at the second surface 208, which is acute around theperimeter of the cavity 210 at the second surface 208. Other wallprofiles having the concave profiles and the acute interior angles 222and 224 are within the scope of the instant example. The first surface206 and the second surface 208 may be cleaned or otherwise treated toprovide a desired bond to a first window and a second window,respectively.

Referring to FIG. 2D, a first window substrate 238 is attached to thebody substrate 230, on the first surface 206. The first window substrate238 may be, for example, a glass wafer. The first window substrate 238may be attached to the body substrate 230 by an anodic bonding processor other process appropriate for providing a desired hermetic seal.

Referring to FIG. 2E, a sensor fluid liquid solution 240 of a sensorfluid material dissolved in a solvent is disposed in the cavity 210. Thesensor fluid liquid solution 240 may include, for example, sensor fluidmaterial of cesium azide dissolved in a solvent of water or alcohol. Thesensor fluid liquid solution 240 may also be disposed in additionalcavities in the body substrate 230. The sensor fluid liquid solution 240may approximately fill the cavity 210. The cell body walls 218 may havehydrophilic surfaces, so that the sensor fluid liquid solution 240 formsa positive meniscus at the cell body walls 218, as depicted in FIG. 2E.

Substantially all of the solvent is subsequently removed from the sensorfluid liquid solution 240 by evaporation, leaving the sensor fluidmaterial in the cavity 210. Evaporation of the solvent may befacilitated by heating the sensor fluid liquid solution 240, reducing anambient pressure over the sensor fluid liquid solution 240, and/orflowing gas over the sensor fluid liquid solution 240 to remove solventvapor.

As the solvent evaporates, the sensor fluid material precipitates fromthe sensor fluid liquid solution 240 onto the cell body walls 218. Thefirst interior angle 222 being acute advantageously enhancesprecipitation of the sensor fluid material onto the first windowsubstrate 238 at a perimeter of the cavity 210, thus avoidingprecipitation in a signal path. The second interior angle 224 beingacute advantageously reduces wicking of the sensor fluid liquid solution240 onto the second surface 208 and hence reduces precipitation of thesensor fluid material onto the second surface 208. If care is taken indisposing the sensor fluid liquid solution 240 into the cavity 210,substantially no precipitation of the sensor fluid material onto thesecond surface 208 is observed.

FIG. 2F depicts the body substrate 230 and attached first windowsubstrate 238 after the solvent is evaporated from the sensor fluidliquid solution 240 of FIG. 2E. The sensor fluid material 216 isprecipitated on the cell body walls 218 and on the first windowsubstrate 238 around a perimeter of the cavity 210.

Referring to FIG. 2G, a second window substrate 242 is attached to thebody substrate 230 on the second surface 208. The second windowsubstrate 242 may have a similar composition and structure as the firstwindow substrate 238. The second window substrate 242 may be attached tothe body substrate 230 by a process similar to that used to attach thefirst window substrate 238. Reducing precipitation of the sensor fluidmaterial 216 on the second surface 208 may advantageously improvereliability of the seal between the second window substrate 242 and thebody substrate 230.

Referring to FIG. 2H, the body substrate 230 with the attached firstwindow substrate 238 and second window substrate 242 are singulated toform a sensor cell 202 of the integrated microfabricated sensor. Thesingulated first window substrate 238 provides a first window 212 of thesensor cell 202. Similarly, the singulated second window substrate 242provides a second window 214 of the cell body 202. The body substrate230 with the attached first window substrate 238 and second windowsubstrate 242 may be singulated by sawing or scribing. Additional sensorcells 202 may be formed by the singulation process. The sensor cell 202is subsequently assembled into the integrated microfabricated sensor,for example as depicted in FIG. 1.

FIG. 3 is an exploded view of another example integrated microfabricatedsensor. The integrated microfabricated sensor 300 includes a sensor cell302, which includes a cell body 304 having a first surface 306 which isflat, and a second surface 308, which is also flat, and is parallel tothe first surface 306. The cell body 304 laterally surrounds a cavity310 that extends from the first surface 306 through the cell body 304 tothe second surface 308. A first window 312 is attached to the cell body304 on the first surface 306, extending across the cavity 310 so as tobe exposed to the cavity 310. A second window 314 is attached to thecell body 304 on the second surface 308, extending across the cavity 310so as to be exposed to the cavity 310. The sensor cell 302 contains asensor fluid material 316 in the cavity 310.

The cell body 304 has cell body walls 318 with concave profiles facingthe cavity 310, so that the cavity 310 is wider in a central region 320c, than at a first boundary region 320 a extending to the first surface306 and at a second boundary region 320 b extending to the secondsurface 308. In the instant example, the cell body walls 318 may besubstantially vertical, that is, within a few degrees of perpendicularto the first surface 306 and the second surface 308, in the centralregion 320 c, with straight facets extending from the central region 320c through the first boundary region 320 a to the first surface 306 andfrom the central region 320 c through the second boundary region 320 bto the second surface 308. The cell body walls 318 have a first interiorangle 322 at the first surface 306 which is acute around a perimeter ofthe cavity 310 at the first surface 306. Similarly, the cell body walls318 have a second interior angle 324 at the second surface 308 which isacute around the perimeter of the cavity 310 at the second surface 308.

The integrated microfabricated sensor 300 includes a signal emitter, notshown in FIG. 3, located outside of the sensor cell 302, configured toemit an input signal through a signal path through the first window 312into the cavity 310. The integrated microfabricated sensor 300 furtherincludes a signal detector, also not shown in FIG. 3, located outside ofthe sensor cell 302, configured to detect an output signal through thesignal path through the second window 314 from the cavity 310. The acutevalues of the first interior angle 322 and the second interior angle 324may provide the advantage discussed in reference to FIG. 1.

FIG. 4A through FIG. 4F are views of an integrated microfabricatedsensor, depicted in successive stages of another example method offormation. Referring to FIG. 4A, a body substrate 430 is provided whichincludes areas for cell bodies, including the cell body 404 of theintegrated microfabricated sensor 400. The body substrate 430 may be,for example, a single crystal wafer. Alternatively, the body substrate430 may be a stack of different materials, bonded together. The bodysubstrate 430 has a first surface 406 which is flat, and a secondsurface 408 which is flat and parallel to the first surface 406.

In the instant example, a first etch mask 432 and a second etch mask 434are formed on the first surface 406 and the second surface 408,respectively. The first etch mask 432 covers the entire first surface406. The second etch mask 434 covers areas on the second surface 408 forcell body walls 418 of the cell bodies and exposes areas for cavities410 of the cell bodies.

The first etch mask 432 and the second etch mask 434 may be formed, forexample, by forming a layer of silicon dioxide 0.5 micrometers to 1micrometer thick concurrently on the first surface 406 and the secondsurface 408, followed by forming a layer of silicon nitride 0.5micrometers to 1 micrometer thick concurrently on the first surface 406and the second surface 408. Hard mask material, such as silicon carbideor amorphous carbon, may be formed over the silicon nitride on thesecond surface 408. Subsequently, a temporary mask, not shown in FIG.4A, may be formed over the second surface 408, and the hard maskmaterials, silicon nitride, and silicon dioxide may be removed in areasexposed by the temporary mask, leaving the second etch mask 434 inplace, followed by removing the temporary mask. Patterning the secondetch mask 434 and leaving the first etch mask 432 unpatterned mayadvantageously reduce fabrication cost and complexity compared topatterning both the first etch mask 432 and the second etch mask 434.Other methods for forming the first etch mask 432 and the second etchmask 434 are within the scope of the instant example.

Referring to FIG. 4B, substrate material of the body substrate 430 isremoved in areas exposed by the second etch mask 434 by a deep reactiveion etch (DRIE) process, for example using halogen radicals 444 such asfluorine, denoted “F” in FIG. 4B, and/or bromine, denoted “Br” in FIG.4B. The DRIE process may alternate etching the body substrate 430 andpassivating cell body walls of the body substrate 430 in the areasexposed by the second etch mask 434, such as the Bosch process.Alternatively, the DRIE process may be a continuous process whichconcurrently etches the body substrate 430 and passivates the cell bodywalls of the body substrate 430. The DRIE process is continued until thefirst etch mask 432 is exposed. A portion or all of the first etch mask432 may be removed by the DRIE process, as indicated in FIG. 4B. Thecell body walls 418 may have substantially vertical profiles after theDRIE process is completed.

Referring to FIG. 4C, the body substrate 430, with the first etch mask432 and the second etch mask 434 in place, is immersed in a wet etchbath 436 which removes substrate material from the cell body walls 418to form faceted profiles at the first surface 406 and at the secondsurface 408, as shown in FIG. 4C. The wet etch bath 436 may include acrystallographic etchant solution as described in reference to FIG. 2B.The first etch mask 432 and the second etch mask 434 are subsequentlyremoved.

FIG. 4D depicts the body substrate 430 after the first etch mask 432 andthe second etch mask 434 of FIG. 4C are removed. The cell body walls 418have concave profiles, so that the cavity 410 is wider in a centralregion 420, approximately midway between the first surface 406 and thesecond surface 408, than at the first surface 406 and at the secondsurface 408. In the instant example, the cell body walls 418 may besubstantially vertical in the central region 420, with straight facetsextending from the central region 420 to the first surface 406 and fromthe central region 420 to the second surface 408. The cell body walls418 have a first interior angle 422 at the first surface 406 which isacute around a perimeter of the cavity 410 at the first surface 406.Similarly, the cell body walls 418 have a second interior angle 424 atthe second surface 408 which is acute around the perimeter of the cavity410 at the second surface 408.

Referring to FIG. 4E, a first window substrate 438 is attached to thebody substrate 430, on the first surface 406. A sensor fluid liquidsolution 440 of a sensor fluid material dissolved in a solvent isdisposed in the cavity 410. The sensor fluid liquid solution 440 mayhave a composition as described in reference to FIG. 2E. The sensorfluid liquid solution 440 may approximately fill the cavity 410.Substantially all of the solvent is subsequently removed from the sensorfluid liquid solution 440 by evaporation, leaving the sensor fluidmaterial in the cavity 410. Evaporation of the solvent may befacilitated by heating the sensor fluid liquid solution 440, reducing anambient pressure over the sensor fluid liquid solution 440, and/orflowing gas over the sensor fluid liquid solution 440 to remove solventvapor. As the solvent evaporates, the sensor fluid material precipitatesfrom the sensor fluid liquid solution 440 onto the cell body walls 418.The first interior angle 422 of FIG. 4D being acute and the secondinterior angle 424 of FIG. 4D being acute may provide the advantagesdiscussed in reference to FIG. 2E.

FIG. 4F depicts the body substrate 430 and attached first windowsubstrate 438 after the solvent is evaporated from the sensor fluidliquid solution 440 of FIG. 4E. The sensor fluid material 416 isprecipitated on the cell body walls 418 and on the first windowsubstrate 438 around a perimeter of the cavity 410. Formation of theintegrated microfabricated sensor 400 is continued by attaching a secondwindow substrate, not shown, to the body substrate 430 at the secondsurface 408, and subsequently singulating the body substrate 430 withthe attached first window substrate 438 and second window substrate.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only and not limitation. Numerous changes to the disclosedembodiments can be made in accordance with the disclosure herein withoutdeparting from the spirit or scope of the invention. Thus, the breadthand scope of the present invention should not be limited by any of theabove described embodiments. Rather, the scope of the invention shouldbe defined in accordance with the following claims and theirequivalents.

What is claimed is:
 1. An integrated microfabricated sensor, comprising:a sensor cell, comprising: a cell body having: a first surface; a secondsurface parallel to the first surface; and cell body walls extendingbetween the first surface and the second surface, the cell body wallshaving a concave profile defining a cavity having: a first boundaryregion along the first surface; a second boundary region along thesecond surface; and a central region between the first surface and thesecond surface, the central region being wider than the first boundaryregion and wider than the second boundary region; a first windowadjacent to the first surface and exposed to the cavity; a second windowadjacent to the second surface and exposed to the cavity; and a sensorfluid material in the cavity; a signal emitter outside of the cavity andproximate to the first window; and a signal detector outside of thecavity and proximate to the second window.
 2. The integratedmicrofabricated sensor of claim 1, wherein the cell body walls have afirst interior angle, extending from the cavity through the cell bodywalls to the first surface, the first interior angle being acute arounda perimeter of the cavity at the first surface.
 3. The integratedmicrofabricated sensor of claim 2, wherein the cell body walls have asecond interior angle, extending from the cavity through the cell bodywalls to the second surface, the second interior angle being acutearound a perimeter of the cavity at the second surface.
 4. Theintegrated microfabricated sensor of claim 1, wherein: the cell bodywalls include crystalline silicon; first facets of the cell body wallsin the first boundary region are aligned along first [111] crystalplanes of the crystalline silicon; and second facets of the cell bodywalls in the second boundary region are aligned along second [111]crystal planes of the crystalline silicon.
 5. The integratedmicrofabricated sensor of claim 4, wherein the first facets and secondfacets meet in the central region.
 6. The integrated microfabricatedsensor of claim 1, wherein the cell body walls have substantiallyvertical profiles in the central region.
 7. The integratedmicrofabricated sensor of claim 1, wherein the sensor fluid materialincludes an alkali metal selected from the group consisting of cesiumand rubidium.
 8. A method, comprising: providing a cell body substratehaving a first surface, and a second surface parallel to the firstsurface; removing material from the cell body substrate to form a cavityextending through the cell body substrate from the first surface to thesecond surface, wherein: a cell body has cell body walls extendingbetween the first surface and the second surface; and the cell bodywalls have a concave profile, so that the cavity is wider in a centralregion, than at the first surface and at the second surface; attaching afirst window substrate to the cell body substrate at the first surface,the first window substrate being exposed to the cavity; disposing asolution comprising a sensor fluid material and a solvent into thecavity on the first window substrate; removing substantially all of thesolvent; and attaching a second window substrate to the cell bodysubstrate at the second surface, the second window substrate beingexposed to the cavity.
 9. The method of claim 8, wherein: the cell bodysubstrate comprises primarily crystalline silicon; the cell body wallshave a first interior angle, extending from the cavity through the cellbody walls to the first surface, the first interior angle being acutearound a perimeter of the cavity at the first surface; and the cell bodywalls have a second interior angle, extending from the cavity throughthe cell body walls to the second surface, the second interior anglebeing acute around a perimeter of the cavity at the second surface. 10.The method of claim 8, further comprising: forming a first etch mask onthe first surface of the cell body substrate, wherein the first etchmask exposes an area on the first surface for the cavity; and forming asecond etch mask on the second surface of the cell body substrate,wherein the second etch mask exposes an area on the second surface forthe cavity; wherein removing the material from the cell body substratecomprises a wet etch process which concurrently removes the materialfrom the cell body in the area exposed by the first mask and removes thematerial from the cell body in the area exposed by the second mask. 11.The method of claim 10, wherein the first etch mask comprises a firstlayer of silicon dioxide formed on the first surface of the cell bodysubstrate, and further comprises a first layer of silicon nitride formedon the first layer of silicon dioxide, and the second etch maskcomprises a second layer of silicon dioxide formed on the second surfaceof the cell body substrate, and further comprises a second layer ofsilicon nitride formed on the second layer of silicon dioxide.
 12. Themethod of claim 11, wherein the first layer of silicon dioxide and thesecond layer of silicon dioxide are formed concurrently, and the firstlayer of silicon nitride and the second layer of silicon nitride areformed concurrently.
 13. The method of claim 8, further comprising:forming a first etch mask on the first surface of the cell bodysubstrate, wherein the first etch mask exposes an area on the firstsurface for the cavity; and forming a second etch mask on the secondsurface of the cell body substrate, wherein the second etch mask coversthe second surface; wherein removing the material from the cell bodysubstrate comprises: performing a deep reactive ion etch (DRIE) processwhich removes the material in an anisotropic manner in the area exposedby the first etch mask; and performing a wet etch process which removesthe material to form the cavity, the wet etch being performed after theDRIE process.
 14. The method of claim 13, wherein the first etch maskcomprises hard mask material selected from the group consisting ofsilicon carbide and amorphous carbon.
 15. The method of claim 8, whereinremoving the material from the cell body comprises a wet etch processwith an aqueous alkaline solution.
 16. The method of claim 8, whereinthe cell body substrate is a silicon wafer having areas for cell bodies,and wherein removing the material from the cell body substrate isperformed concurrently with removing material from the cell bodysubstrate to form a cavity in each of the cell bodies.
 17. The method ofclaim 8, wherein the sensor fluid material comprises an alkali metalsalt and the solvent comprises a fluid selected from the groupconsisting of water and alcohol.
 18. The method of claim 17, wherein thealkali metal salt comprises cesium azide.
 19. The method of claim 8,wherein: the first window substrate comprises a first glass layer;attaching the first window substrate to the cell body substratecomprises a first anodic bonding process; the second window substratecomprises a second glass layer; and attaching the second windowsubstrate to the cell body substrate comprises a second anodic bondingprocess.
 20. A method, comprising: providing a cell body substratehaving a first surface, and a second surface parallel to the firstsurface; removing material from the cell body substrate to form a cavityextending through the cell body substrate from the first surface to thesecond surface, wherein: a cell body has cell body walls extendingbetween the first surface and the second surface; and the cell bodywalls have a concave profile, so that the cavity is wider in a centralregion, than at the first surface and at the second surface; attaching afirst window substrate to the cell body substrate at the first surface,the first window substrate being exposed to the cavity; disposing asolution comprising a sensor fluid material and a solvent into thecavity on the first window substrate; removing substantially all of thesolvent; attaching a second window substrate to the cell body substrateat the second surface, the second window substrate being exposed to thecavity; forming a first etch mask on the first surface of the cell bodysubstrate, wherein the first etch mask exposes an area on the firstsurface for the cavity; forming a second etch mask on the second surfaceof the cell body substrate, wherein the second etch mask exposes an areaon the second surface for the cavity, wherein removing the material fromthe cell body substrate comprises a wet etch process which concurrentlyremoves the material from the cell body in the area exposed by the firstmask and removes the material from the cell body in the area exposed bythe second mask; the first etch mask comprises a first layer of silicondioxide formed on the first surface of the cell body substrate, andfurther comprises a first layer of silicon nitride formed on the firstlayer of silicon dioxide, and the second etch mask comprises a secondlayer of silicon dioxide formed on the second surface of the cell bodysubstrate, and further comprises a second layer of silicon nitrideformed on the second layer of silicon dioxide; and the first layer ofsilicon dioxide and the second layer of silicon dioxide are formedconcurrently, and the first layer of silicon nitride and the secondlayer of silicon nitride are formed concurrently.